Transmission device and reception device

ABSTRACT

A transmission device ( 100 ) outputs, to a reception device ( 200 ), a stored amount of packets in a first transmission buffer ( 105 ) just before a certain packet has been written to the first transmission buffer. A first reception buffer amount read unit ( 206 ) in the reception device reads a stored amount of packets in a first reception buffer just before the certain packet has been read from the first reception buffer. A storage amount addition unit ( 207 ) adds the stored amount of packets in the first transmission buffer and the stored amount of packets in the first reception buffer, and a correction unit ( 208 ) adjusts the frequency of a variable frequency oscillator in a reception timestamp timer  209  so that the resulting added value is a constant value.

TECHNICAL FIELD

The present invention relates to technology for performing synchronoustransmission between a transmission device and a reception device, suchas MPEG2 transport stream (MPEG2 TS) transmission.

Synchronous transmission refers to transmission in which the average bitrate at which packets are input to a transmission device is equal to theaverage bit rate at which packets are output from a reception device,and the time-base jitter of packets is within a predetermined range.

BACKGROUND ART

Conventionally, there is technology that realizes synchronoustransmission by merely performing transmission jitter control with theuse of a buffer included in a reception device, under the condition thatthe input bit rate at which packets are input to a transmission deviceand the transmission bit rate of packet on a transmission line are botha constant bit rate (CBR) and both the same value (see patent document1).

In the embodiments of the present invention, “constant bit rate” meansthat the average bit rate per fixed period is a constant value, and“variable bit rate” means that the average bit rate per certain fixedperiod varies.

The following describes an outline of the technology disclosed in patentdocument 1 for realizing conventional synchronous transmission withreference to FIG. 17. FIG. 17 shows the structure of a reception devicethat realizes such conventional synchronous transmission.

A reception device 1000 includes an input terminal 1001 that receives aninput of a packet from a transmission line, a conditioning circuit 1002with a transmission jitter buffer (not depicted) provided therein, asystem decoder 1003, a timestamp acquisition circuit 1004, a PLL (PhaseLocked Loop) circuit 1005, an output terminal 1006 for externallyoutputting packets, and an output terminal 1007 for externallyoutputting a system clock.

In the following, BuffRx(t) is a stored amount function that expressesthe amount of packets that are stored in the transmission jitter bufferin the conditioning circuit 1002 at time t. G(t) is a rate function thatexpresses the input bit rate at which a packet is input to thetransmission jitter buffer at time t, and F(t) is a rate function thatexpresses the output bit rate at which a packet is output from thetransmission jitter buffer at time t.

The rate function G(t) is 0 until time T0, and c (c being a constantvalue) at or after time T0, and the rate function F(t) is 0 until timeT1, and c (c being a constant value) at or after time T1 (FIG. 18A).

The stored amount function BuffRx(t) at time t, which is at or aftertime T1, is expressed by the following expression (1)

$\begin{matrix}{{{Expression}\mspace{14mu} (1)}\mspace{596mu}} & \; \\{{{BuffRx}(t)} = {{{\int_{0}^{t}{{G(t)}{t}}} - {\int_{0}^{t}{{F(t)}{t}}}} = {{{\int_{T\; 0}^{T\; 1}{{G(t)}{t}}} + {\int_{T\; 1}^{t}{{G(t)}{t}}} - {\int_{T\; 1}^{t}{{F(t)}{t}}}} = {{{\int_{T\; 0}^{T\; 1}{c{t}}} + {\int_{T\; 1}^{t}{c{t}}} - {\int_{T\; 1}^{t}{c{t}}}} = {{\int_{T\; 0}^{T\; 1}{c{t}}} = {{constant}\mspace{14mu} {value}}}}}}} & (1)\end{matrix}$

In expression (1), since the data amount of packets input to the bufferfrom time T1 to time t is the same as the data amount of packets outputfrom the buffer during the same period, expression (1) shows that thestored amount function BuffRx(t) is constant from time T1 onward (FIG.18B). In other words, the stored amount of packets in the buffer isconstant.

In consideration of this, the conventional technology realizessynchronous transmission by controlling the speed at which packets areoutput from the buffer so that the stored amount of packets in thebuffer is constant.

Patent document 1: Japanese Patent Application Publication No. H8-139704

DISCLOSURE OF THE INVENTION Problems Solved by the Invention

However, the conventional technology cannot realize synchronoustransmission between transmission and reception devices in cases ofcommunication such as when the stored amount of packets in the bufferincluded in the reception device is not constant. Examples of suchcommunication include (1) when the input bit rate at which packets areinput to the transmission device is a variable bit rate (VBR) and thetransmission bit rate on the transmission line is a constant bit rate,(2) when the input bit rate at which packets are input to thetransmission device is a constant bit rate and the transmission bit rateon the transmission line is a variable bit rate, and (3) when the inputbit rate at which packets are input to the transmission device and thetransmission bit rate on the transmission line are both variable bitrates.

The following describes how the stored amount of packets in the bufferof a reception device is not constant in the above case (2), withreference to FIGS. 19A and 19B.

In this case, the rate function G(t) is 0 until time T0, and a variablevalue at or after time T0, and the rate function F(t) is 0 until timeT1, and c (c being a constant value) at or after T1 (FIG. 19A).

The stored amount function BuffRx(t) at time t, which is at or aftertime T1, is expressed by the following expression (2)

$\begin{matrix}{{{Expression}\mspace{14mu} (2)}\mspace{596mu}} & \; \\{{{BuffRx}(t)} = {{{\int_{0}^{t}{{G(t)}{t}}} - {\int_{0}^{t}{{F(t)}{t}}}} = {{{\int_{T\; 0}^{T\; 1}{{G(t)}{t}}} + {\int_{T\; 1}^{t}{{G(t)}{t}}} - {\int_{T\; 1}^{t}{{F(t)}{t}}}} = {{{\int_{T\; 0}^{T\; 1}{{G(t)}{t}}} + {\int_{T\; 1}^{t}{{G(t)}{t}}} - {\int_{T\; 1}^{t}{c{t}}}} = {{variable}\mspace{14mu} {value}}}}}} & (2)\end{matrix}$

In expression (2), since the data amount of packets input to the bufferfrom time T1 to time t is not necessarily the same as the data amount ofpackets output from the buffer during the same period, expression (2)shows that the stored amount function BuffRx(t) is not constant fromtime T1 onward (FIG. 19B). In other words, the stored amount of packetsin the buffer is not constant.

In view of the above, an aim of the present invention is to provide atransmission device, a reception device, a transmission method, and areception method that, by utilizing the stored amount of packets in thebuffers included in the transmission and reception devices, realizesynchronous transmission between the transmission and reception deviceseven when either of, or both, the input bit rate at which packets areinput to the transmission device and the transmission bit rate on thetransmission line is a variable bit rate.

Means to Solve the Problems

In order to achieve the above aim, the present invention is atransmission device including: a transmission counter unit operable tocount a constant frequency clock signal; a first transmission bufferunit operable to temporarily store a packet input thereto, and outputthe stored packet; and a transmission unit operable to (i) transmit thepacket stored in the first transmission buffer unit and transmissioncounter information, the transmission counter information indicating acount value of the transmission counter unit when the packet was inputto the transmission device, and (ii) transmit first transmission storageinformation via a transmission line, the first transmission storageinformation specifying a stored amount of packets in the firsttransmission buffer unit at a first timing pertaining to firstprocessing performed on the packet with respect to the firsttransmission buffer unit.

The present invention is also a transmission method of (i) transmittinga packet stored in a first transmission buffer unit and transmissioncounter information indicating a count value of a transmission counterunit when the packet was input to a transmission device, and (ii)transmitting, via a transmission line, first transmission storageinformation for specifying a stored amount of packets in the firsttransmission buffer unit at a first timing pertaining to firstprocessing performed on the packet with respect to the firsttransmission buffer unit.

The present invention is also a reception device including: a receptionunit operable to receive, via a transmission line and from atransmission device including a transmission counter unit that counts aconstant frequency clock signal and a first transmission buffer unit,(i) a packet, (ii) transmission counter information indicating a countvalue of the transmission counter unit when the packet was input to thetransmission device, and (iii) first transmission storage informationfor specifying a stored amount of packets in the first transmissionbuffer unit at a first timing pertaining to first processing performedon the packet with respect to the first transmission buffer unit; areception counter unit operable to count a clock signal at a countingspeed that is variable; a first reception buffer unit operable totemporarily store the packet; a correction unit operable to correct thecounting speed of the reception counter unit based on the firsttransmission storage information and first reception storageinformation, the first reception storage information specifying a storedamount of packets in the first reception buffer unit at a second timingpertaining to second processing performed on the packet with respect tothe first reception buffer unit; and a first output control unitoperable to control output of the packet from the first reception bufferunit based on the transmission counter information received by thereception unit and reception counter information indicating a countvalue of the reception counter unit.

The present invention is also a reception method used in a receptiondevice which includes a reception counter unit that counts a clocksignal at a counting speed that is variable, and a first receptionbuffer unit that temporarily stores a received packet, including thesteps of: receiving, via a transmission line and from a transmissiondevice including a transmission counter unit that counts a constantfrequency clock signal and a first transmission buffer unit, (i) thepacket, (ii) transmission counter information indicating a count valueof the transmission counter unit when the packet was input to thetransmission device, and (iii) first transmission storage informationfor specifying a stored amount of packets in the first transmissionbuffer unit at a first timing pertaining to first processing performedon the packet with respect to the first transmission buffer unit;correcting the counting speed of the reception counter unit based on thefirst transmission storage information and first reception storageinformation, the first reception storage information specifying a storedamount of packets in the first reception buffer unit at a second timingpertaining to second processing performed on the packet with respect tothe first reception buffer unit; and performing first output control tocontrol output of the packet from the first reception buffer unit basedon the transmission counter information received in the receiving stepand reception counter information indicating a count value of thereception counter unit.

EFFECTS OF THE INVENTION

A combination of the transmission device and reception device of thepresent invention, or a combination of the transmission method andreception method of the present invention, enables adjusting the countspeeds of the counters in the transmission and reception devices byutilizing the stored amount of packets in the first transmission bufferunit of the transmission device and the stored amount of packets in thesecond reception buffer unit of the reception device. This enablessynchronous transmission between transmission and reception devices,even in the case of communication between transmission and receptiondevices in which synchronous communication could not be realized byutilizing solely the stored amount of packets in the buffer included inthe reception device (i.e., the bit rate at which packets are input tothe transmission device is a variable bit rate, or the transmission bitrate on the transmission line is a variable bit rate).

In the transmission device, the first transmission storage informationmay be the stored amount of packets in the first transmission bufferunit.

In the reception device, the first transmission storage information maybe the stored amount of packets in the first transmission buffer unit,and the first reception storage information may be the stored amount ofpackets in the first reception buffer unit.

This structure enables the reception device to easily specify the storedamount of packets in the first transmission buffer unit.

The transmission device may further include a counter informationattachment unit operable to attach the transmission counter informationto the packet, wherein the transmission unit may perform thetransmission of the packet and the transmission counter information bytransmitting the packet to which the transmission counter informationhas been attached by the counter information attachment unit.

This structure enables the reception device to easily specify thecounter information that corresponds to the packet and that is from whenthe packet was input to the transmission device.

The transmission device may further include a storage informationattachment unit operable to attach the first transmission storageinformation to the packet, wherein the transmission unit may perform thetransmission of the packet and the first transmission storageinformation by transmitting the packet to which the first transmissionstorage information has been attached by the storage informationattachment unit.

This structure enables the reception device to easily specify the firsttransmission storage information that corresponds to the packet.

The transmission device may further include a number attachment unitoperable to sequentially attach numbers to packets that are to betransmitted by the transmission unit, in an order in which the packetswere input to the transmission device.

In the reception device, the reception unit may be further operable toreceive, from the transmission device, sequential numbers that have beenassigned to packets in the transmission device in an order in which thepackets were input to the transmission device, the reception device mayfurther include: a detection unit operable to detect a count of packetsthat were not received by the reception unit, based on the sequentialnumbers received by the reception unit, and the correction unit maycorrect the counting speed of the reception counter unit so as to obtaina constant value for a sum of the stored amount of packets in the firsttransmission buffer unit, the stored amount of packets in the firstreception buffer unit, and a missing packet amount that is based on thedetected count of packets that were not received by the reception unit.

A combination of the above transmission device and reception deviceenables realizing synchronous transmission between the transmission andreception devices even if a packet is lost on the transmission line.

In the transmission device, the first transmission buffer unit mayoutput the stored packet at a constant bit rate, the transmission devicemay further include: a second transmission buffer unit operable totemporarily store the packet output from the first transmission bufferunit, the first timing pertaining to the first processing may be atiming pertaining to input processing, and the transmission unit may befurther operable to transmit second transmission storage information,the second transmission storage information specifying a stored amountof packets in the second transmission buffer unit at a second timingpertaining to second processing, the second processing being processingfor outputting the packet from the second transmission buffer unit.

In the reception device, the first timing pertaining to the firstprocessing may be a timing pertaining to input processing, the receptionunit may be further operable to receive second transmission storageinformation from the transmission device which further includes a secondtransmission buffer unit that temporarily stores the packet output fromthe first transmission buffer unit at a constant bit rate and outputsthe stored packet, the second transmission storage informationspecifying a stored amount of packets in the second transmission bufferunit at a timing pertaining to processing for outputting the packet fromthe second transmission buffer unit, and the reception device mayfurther include: a second reception buffer unit operable to temporarilystore the packet and output the stored packet to the first receptionbuffer unit; and a second output control unit operable to control theoutput of the packet from the second reception buffer unit, based on thesecond transmission storage information and second reception storageinformation, the second reception storage information specifying astored amount of packets in the second reception buffer unit at a timingpertaining to processing for inputting the packet to the secondreception buffer unit.

A combination of the above transmission device and reception deviceenables realizing synchronous transmission between the transmission andreception devices even if the input bit rate at which packets are inputto the transmission apparatus and the transmission bit rate on thetransmission line are both variable bit rates.

In the transmission device, the first transmission buffer unit mayoutput the stored packet at a constant bit rate, the transmission devicemay further include: a second transmission buffer unit operable totemporarily store the packet output from the first transmission bufferunit, and the first timing pertaining to the first processing is atiming pertaining to input processing.

In the reception device, the first timing pertaining to the firstprocessing may be a timing pertaining to input processing, thetransmission device may further include a second transmission bufferunit that temporarily stores the packet output from the firsttransmission buffer unit at a constant bit rate and outputs the storedpacket, and the reception device may further include: a second receptionbuffer unit operable to temporarily store the packet and output thestored packet to the first reception buffer unit; and a second outputcontrol unit operable to control the output of the packet from thesecond reception buffer unit based on (i) the first transmission storageinformation for specifying the stored amount of packets in the firsttransmission buffer unit at the first timing pertaining to firstprocessing performed on the packet with respect to the firsttransmission buffer unit, and (ii) the first reception storageinformation for specifying the stored amount of packets in the firstreception buffer unit at the second timing pertaining to secondprocessing performed on the packet with respect to the first receptionbuffer unit.

A combination of the above transmission device and reception deviceenables realizing synchronous transmission between the transmission andreception devices even if the input bit rate at which packets are inputto the transmission apparatus and the transmission bit rate on thetransmission line are both variable bit rates.

The transmission device may further include a second transmission bufferunit operable to temporarily store the input packet, and transmit thestored packet to the first transmission buffer unit at a constant bitrate.

The present invention is also a reception device including: a receptionunit operable to receive, via a transmission line and from atransmission device including a transmission counter unit that counts aconstant frequency clock signal, a first transmission buffer unit thattemporarily stores a packet and outputs the stored packet at a constantbit rate, and a second transmission buffer unit that temporarily storesthe packet output from the first transmission buffer unit, (i) thepacket, (ii) transmission counter information indicating a count valueof the transmission counter unit when the packet was input to thetransmission device, and (iii) transmission storage information forspecifying a stored amount of packets in the second transmission bufferunit at a timing pertaining to processing for outputting the packet fromthe second transmission buffer unit; a reception counter unit operableto count a clock signal at a counting speed that is variable; a firstreception buffer unit operable to temporarily store the packet; a secondreception buffer unit operable to temporarily store the packet andoutput the stored packet to the first reception buffer unit; acorrection unit operable to correct the counting speed of the receptioncounter unit based on the transmission storage information and receptionstorage information, the reception storage information specifying astored amount of packets in the second reception buffer unit at a timingpertaining to processing for inputting the packet to the secondreception buffer unit; a first output control unit operable to controloutput of the packet from the first reception buffer unit, based on thetransmission counter information received by the reception unit andreception counter information indicating a count value of the receptioncounter unit; and a second output control unit operable to control theoutput of the packet from the second reception buffer unit, based on thetransmission storage information and the reception storage information.

A combination of the above transmission device and reception deviceenables realizing synchronous transmission between the transmission andreception devices even if the input bit rate at which packets are inputto the transmission apparatus and the transmission bit rate on thetransmission line are both variable bit rates.

The present invention is also a reception method used in a receptiondevice which includes a reception counter unit that counts a clocksignal at a counting speed that is variable, a first reception bufferunit that temporarily stores a packet, and a second reception bufferunit that temporarily stores the packet and outputs the stored packet tothe first reception buffer unit, including the steps of: receiving, viaa transmission line and from a transmission device including atransmission counter unit that counts a constant frequency clock signal,a first transmission buffer unit that temporarily stores the packet andoutputs the stored packet at a constant bit rate, and a secondtransmission buffer unit that temporarily stores the packet output fromthe first transmission buffer unit, (i) the packet, (ii) transmissioncounter information indicating a count value of the transmission counterunit when the packet was input to the transmission device, and (iii)transmission storage information for specifying a stored amount ofpackets in the second transmission buffer unit at a timing pertaining toprocessing for outputting the packet from the second transmission bufferunit; correcting the counting speed of the reception counter unit basedon the transmission storage information and reception storageinformation, the reception storage information specifying a storedamount of packets in the second reception buffer unit at a timingpertaining to processing for inputting the packet to the secondreception buffer unit; performing first output control to control outputof the packet from the first reception buffer unit, based on thetransmission counter information received in the receiving step andreception counter information indicating a count value of the receptioncounter unit; and performing second output control to control the outputof the packet from the second reception buffer unit, based on thetransmission storage information and the reception storage information.

The above reception method enables realizing synchronous transmissionbetween the transmission and reception devices even if the input bitrate at which packets are input to the transmission apparatus and thetransmission bit rate on the transmission line are both variable bitrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of a synchronous transmission system ofembodiment 1;

FIGS. 2A and 2B show changes in bit rate and stored amount of packets ina buffer over time in the synchronous transmission system of FIG. 1;

FIG. 3 shows changes in packet structure over time in the synchronoustransmission system of FIG. 1;

FIG. 4 shows the structure of a transmission timestamp timer shown inFIG. 1;

FIG. 5 shows the structure of a reception timestamp timer shown in FIG.1;

FIG. 6 shows the structure of a synchronous transmission system ofembodiment 2;

FIG. 7 shows the structure of a missing packet detection unit shown inFIG. 6;

FIG. 8 shows the structure of a synchronous transmission system ofembodiment 3;

FIGS. 9A and 9B show changes in bit rate and stored amount of packets ina buffer over time in the synchronous transmission system of FIG. 8;

FIG. 10 show changes in packet structure over time in the synchronoustransmission system of FIG. 8;

FIG. 11 shows the structure of a synchronous transmission system ofembodiment 4;

FIG. 12 shows the structure of a synchronous transmission system ofembodiment 5;

FIG. 13 shows changes in packet structure over time in the synchronoustransmission system of FIG. 12;

FIG. 14 shows the structure of an output rate control unit shown in FIG.12;

FIG. 15 shows the structure of a synchronous transmission system ofembodiment 6;

FIG. 16 shows the structure of a synchronous transmission system ofembodiment 7;

FIG. 17 shows the structure of a reception device that realizesconventional synchronous transmission;

FIGS. 18A and 18B show changes in input/output bit rate of packets to abuffer and stored amount of packets in a buffer over time inconventional technology; and

FIGS. 19A and 19B show changes in input/output bit rate of packets to abuffer and stored amount of packets in a buffer over time, to illustratea problem in conventional technology.

DESCRIPTION OF THE CHARACTERS

-   -   100 transmission device    -   101 input terminal    -   102 transmission timestamp timer    -   103 timestamp attachment unit    -   104 first transmission buffer amount attachment unit    -   105 first transmission buffer    -   106 first transmission buffer amount buffer    -   107 transmission processing device    -   200 reception device    -   201 reception processing device    -   202 separation unit    -   203 first reception buffer    -   204 timestamp timer    -   205 first transmission buffer amount buffer    -   206 first reception buffer amount read unit    -   207 stored amount addition unit    -   208 correction unit    -   209 reception timestamp timer    -   210 comparison unit    -   211 packet read unit    -   300 transmission line

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

The following describes a synchronous transmission system pertaining toembodiment 1 of the present invention that realizes synchronoustransmission between a transmission device and a reception device withreference to the drawings.

The present embodiment and the later-described embodiment 2 are directedat a case in which the input bit rate at which packets are input to thetransmission device is a variable bit rate, and the transmission bitrate on the transmission line is a constant bit rate.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 1. FIG. 1 showsthe structure of the synchronous transmission system of the presentembodiment.

In the synchronous transmission system, packets are transmitted andreceived between a transmission device 100 and a reception device 200via a transmission line 300. The transmission line 300 is a transmissionline in, for example, an ATM (Asynchronous Transfer Mode) network.

Stored Amount of Packets

Prior to the description of the constituent elements shown in FIG. 1,the following describes the stored amount of packets in a firsttransmission buffer 105 and a first reception buffer 203 shown in FIG.1, with reference to FIGS. 2A and 2B. FIGS. 2A and 2B show changes inbit rate and stored amount of packets over time in the synchronoustransmission system of FIG. 1.

The input bit rate at which packets are input to the first transmissionbuffer 105 of the transmission device 100 is a variable bit rate, and isexpressed by a rate function f1(t). Also, the transmission bit rate onthe transmission line is a constant bit rate, and is expressed by a ratefunction g1(t).

In synchronous transmission, the output bit rate at which packets areoutput from the first reception buffer 203 of the reception device 200is controlled so as to be equal to the input bit rate at which packetsare input to the first transmission buffer 105. For this reason, letting“delay” express a delay time from when a packet is input to thetransmission device 100 until when a packet is output from the receptiondevice 200, the rate function f1(t-delay) expresses the output bit rateat which packets are output from the first reception buffer 203.

Here, (1) packet writing to the first transmission buffer 105 is begunat time 0, (2) packet reading from the first transmission buffer 105 andpacket writing to the first reception buffer 203 are begun at time t0,and (3) packet reading from the first reception buffer 203 is begun attime t1 (=delay).

In this case, the rate function f1(t) is a variable value, and thefunction g1(t) is 0 until time t0, and c (c being a constant value) atand after time t0 (FIG. 2A). Note that the function f1(t-delay) is 0until time t1 (=delay), and a variable value at and after time t1.

Letting BuffTx1(t) be a stored amount function expressing the storedamount of packets in the first transmission buffer 105 at time t, thefollowing expression (3) indicates the stored amount function BuffTx1(t)at time t, which is at or after time t1 (FIG. 2B).

$\begin{matrix}{{{Expression}\mspace{14mu} (3)}\mspace{596mu}} & \; \\{{{BuffTx}\; 1(t)} = {{{\int_{0}^{t}{f\; 1(t){t}}} - {\int_{0}^{t}{g\; 1(t){t}}}} = {{\int_{0}^{t}{f\; 1(t){t}}} - {\int_{t\; 0}^{t\;}{c{t}}}}}} & (3)\end{matrix}$

Letting BuffRx1(t) be a stored amount function expressing the storedamount of packets in the first reception buffer 203 at time t, thefollowing expression (4) indicates the stored amount function BuffRx1(t)at time t, which is at or after time t1 (FIG. 2B)

$\begin{matrix}{{{Expression}\mspace{14mu} (4)}\mspace{596mu}} & \; \\{{{BuffRx}\; 1(t)} = {{{\int_{0}^{t}{g\; 1(t){t}}} - {\int_{0}^{t}{f\; 1\left( {t - {delay}} \right){t}}}} = {{\int_{t\; 0}^{t}{c{t}}} - {\int_{delay}^{t}{f\; 1\left( {t - {delay}} \right){t}}}}}} & (4)\end{matrix}$

Based on expressions (3) and (4), the following expression (5) indicatesa sum of the stored amount of packets in the first transmission buffer105 at time t, which is at or after time t1, and the stored amount ofpackets in the first reception buffer 203 at time t+delay.

$\begin{matrix}{{{Expression}\mspace{14mu} (5)}\mspace{596mu}} & \; \\{{{{BuffTx}\; 1(t)} + {{BuffRx}\; 1\left( {t + {delay}} \right)}} = {{{\int_{0}^{t}{f\; 1(t){t}}} - {\int_{t\; 0}^{t}{c{t}}} + {\int_{t\; 0}^{t + {delay}}{c{t}}} - {\int_{delay}^{t + {delay}}{f\; 1\left( {t - {delay}} \right){t}}}} = {{{\int_{0}^{t}{f\; 1(t){t}}} + {\int_{t}^{t + {delay}}{c\; {t}}} - {\int_{0}^{t}{f\; 1(t){t}}}} = {{\int_{t}^{t + {delay}}{c{t}}} = {{constant}\mspace{14mu} {value}}}}}} & (5)\end{matrix}$

As shown above, the sum of the stored amount of packets in the firsttransmission buffer 105 at time t, which is at or after time t1, and thestored amount of packets in the first reception buffer 203 at timet+delay is a constant value (FIG. 2B).

In other words, a constant value is obtained for the sum of the storedamount of packets in the first transmission buffer 105 just before acertain packet is written to the first transmission buffer 105 and thestored amount of packets in the first reception buffer 203 just beforethe certain packet is read from the first reception buffer 203. Also, aconstant value is obtained for the sum of the stored amount of packetsin the first transmission buffer 105 just after a certain packet hasbeen written to the first transmission buffer 105 and the stored amountof packets in the first reception buffer 203 just after the certainpacket has been read from the first reception buffer 203.

In the present embodiment, synchronous transmission between thetransmission device 100 and reception device 200 is realized byutilizing the above relationship in which the sum of the stored packetamounts is constant.

The present embodiment makes use of the sum of the stored amount ofpackets in the first transmission buffer 105 just before a certainpacket is written to the first transmission buffer 105 and the storedamount of packets in the first reception buffer 203 just before thecertain packet is read from the first reception buffer 203.

Note that it is acceptable to use the sum of the stored amount ofpackets in the first transmission buffer 105 just after a certain packethas been written to the first transmission buffer 105 and the storedamount of packets in the first reception buffer 203 just after thecertain packet has been read from the first reception buffer 203.

Although it is desirable to use either of the above sums to realizesynchronous transmission, when a sufficient amount of packets is storedin the first transmission buffer 105 and the first reception buffer 203,there may be a slight difference in the timing at which the storedamounts of the first transmission buffer 105 and the first receptionbuffer 203 are acquired.

Transmission Device

The transmission device 100 includes an input terminal 101, atransmission timestamp timer 102, a timestamp attachment unit 103, afirst transmission buffer amount attachment unit 104, the firsttransmission buffer 105, a first transmission buffer amount read unit106, and a transmission processing unit 107.

When a packet is input to the input terminal 101, the input packet isoutput to the timestamp attachment unit 103.

The transmission timestamp timer 102 counts a constant frequency clocksignal and outputs a count value. An exemplary structure of thetransmission timestamp timer 102 is described later with reference toFIG. 4.

The timestamp attachment unit 103 receives the input packet and attachesa count value, which is supplied from the transmission timestamp timer102, to the beginning of the received packet as a timestamp. Note thatin the present embodiment, although the count value is directly used asthe timestamp, the count value may be substituted with a time, andinformation indicating the substituted time may be used as thetimestamp.

The first transmission buffer amount attachment unit 104 attaches alater-described first transmission buffer amount, which is input fromthe first transmission buffer amount read unit 106, to the beginning ofthe packet having the timestamp attached thereto.

The first transmission buffer 105 is constituted from a FIFO (First InFirst Out) buffer. The first transmission buffer 105 temporarily storesinput packets, and outputs the stored packets at a constant bit rate.Note that the first transmission buffer 105 joins a plurality of packetstogether and outputs the group of packets (hereinafter, called the“joined packet”) in order to increase transmission efficiency.

The first transmission buffer amount read unit 106 reads the storedamount of packets in the first transmission buffer 105 (hereinafter,called the “first transmission buffer amount”), and outputs the firsttransmission buffer amount to the first transmission buffer amountattachment unit 104. Here, the first transmission buffer amount is thestored amount of packets in the first transmission buffer 105 justbefore the writing, to the first transmission buffer 105, of the packetto which the first transmission buffer amount is to be attached by thefirst transmission buffer amount attachment unit 104.

In order to facilitate processing in the reception device 200, the firsttransmission buffer amount is the stored amount of data in the firsttransmission buffer 105 excluding the data amounts of the timestamp andthe first transmission buffer amount itself.

The first transmission buffer amount can be calculated in, for example,the following way. The data size of the timestamp and the firsttransmission buffer amount are assumed to be fixed data sizes. The sumof the data sizes of the timestamp and the first transmission bufferamount is multiplied by the number of packets stored in the firsttransmission buffer 105 (a value equaling the number of input packetsminus the number of output packets). The value resulting from themultiplication is subtracted from the actual stored amount of the firsttransmission buffer 105. The value resulting from the subtraction is thefirst transmission buffer amount.

The transmission processing unit 107 performs modulation processing,protocol processing, address header processing, etc. suitable fortransmission. A description of the modulation processing, protocolprocessing, address header processing, etc. has been omitted since theyare not directly related to the present invention. It should be notedthat the present invention can be applied to any kind of transmission,regardless of the modulation processing, protocol processing, addressheader processing, etc.

In the above-described transmission device 100, when a packet is inputto the input terminal 101 (N101 of FIG. 3), the timestamp attachmentunit 103 attaches a count value supplied from the transmission timestamptimer 102 to the beginning of the input packet as a timestamp (N102),and furthermore, the first transmission buffer amount attachment unit104 attaches a first transmission buffer amount read by the firsttransmission buffer amount read unit 106 to the beginning of the packethaving the timestamp attached thereto (N103). The packet having attachedthe timestamp and first transmission buffer amount attached thereto isstored in the first transmission buffer 105.

The first transmission buffer 105 outputs the joined packet (N104). Thetransmission processing unit 107 performs predetermined processing onthe joined packet, and the resulting packet is sent to the transmissionline 300.

Transmission Timestamp Timer

The following describes the structure of the transmission timestamptimer 102 included in the transmission device 100 shown in FIG. 1, withreference to FIG. 4. FIG. 4 shows the structure of the transmissiontimestamp timer 102 shown in FIG. 1.

The transmission timestamp timer 102 includes an oscillator 102 a, acounter 102 b, and an output terminal 102 c.

The oscillator 102 a is a constant frequency oscillator, and isconstituted so as to generate a constant frequency clock signal with useof quartz or the like. The oscillation frequency is set high enough thatthe residual time-base jitter of a post-cycle recovery packet is withina predetermined amount.

The counter 102 b counts the constant frequency clock signal generatedby the oscillator 102 a, and externally outputs a count value via theoutput terminal 102 c. The count value output via the output terminal102 c is supplied to the timestamp attachment unit 103.

Reception Device

The reception device 200 performs time-base recovery processing, ratecontrol processing and the like, which are conditions of synchronoustransmission. Note that time-base recovery processing is processing forkeeping the time-base jitter of packets within a predetermined range.Also, rate control processing is processing for equalizing the averagebit rate of transmitted packets between transmission and receptiondevices.

The reception device 200 includes a reception processing unit 201, aseparation unit 202, the first reception buffer 203, a timestamp buffer204, a first transmission buffer amount buffer 205, a first receptionbuffer amount read unit 206, a stored amount addition unit 207, acorrection unit 208, a reception timestamp timer 209, a comparison unit210, a packet read unit 211, and an output terminal 212.

Upon receiving a packet via the transmission line 300, the receptionprocessing unit 201 performs processing that is opposite from theprocessing performed by the transmission processing unit 107, andrecovers the joined packet. A description of the processing performed bythe reception processing unit 201 has been omitted since it does notdirectly relate to the present invention.

The separation unit 202 separates the joined packet received from thereception processing unit 201 into a plurality of first transmissionbuffer amounts, a plurality of timestamps, and a plurality of packets.The separation unit 202 outputs the first transmission buffer amounts,timestamps, and packets to the first transmission buffer amount buffer205, timestamp buffer 204, and first reception buffer 203 respectively.

The first reception buffer 203 is constituted from a FIFO buffer, andtemporarily stores input packets. Packets output from the firstreception buffer 203 are externally sent via the output terminal 212.

The timestamp buffer 204 is constituted from a FIFO buffer, andtemporarily stores input timestamps.

The first transmission buffer amount buffer 205 is constituted from aFIFO buffer, and temporarily stores input first transmission bufferamounts.

The first reception buffer amount read unit 206 reads the stored amountof packets in the first reception buffer 203 (hereinafter, called the“first reception buffer amount”), and outputs the read first receptionbuffer amount to the stored amount addition unit 207. The firstreception buffer amount is the stored amount of packets in the firstreception buffer 203 just before the reading from the first receptionbuffer 203 of the packet having attached thereto the first transmissionbuffer amount that was output to the stored amount addition unit 207 bythe first transmission buffer amount buffer 205.

The stored amount addition unit 207 adds the first transmission bufferamount that was retrieved from the first transmission buffer amountbuffer 205 and the first reception buffer amount that was received fromthe first reception buffer amount read unit 206. Note that since boththe first reception buffer 203 and the first transmission buffer amountbuffer 205 are constituted from FIFO buffers, the first transmissionbuffer amount retrieved from the first transmission buffer amount buffer205 is the first transmission buffer amount that is attached to thepacket to be read from the first reception buffer 203.

The correction unit 208 controls the counting speed of the receptiontimestamp timer 209 so that the added value received as input from thestored amount addition unit 207 is constant.

If the added value increases, the counting speed of the receptiontimestamp timer 209 has become slower than the counting speed of thetransmission timestamp timer 102. The correction unit 208 thereforeoutputs a control signal instructing the reception timestamp timer 209to increase the counting speed.

If the added value decreases, the counting speed of the receptiontimestamp timer 209 has become faster than the counting speed of thetransmission timestamp timer 102. The correction unit 208 thereforeoutputs a control signal instructing the reception timestamp timer 209to decrease the counting speed.

The reception timestamp timer 209 decreases the counting speed uponreceiving an input of the control signal instructing a reduction incounting speed, and increases the counting speed upon receiving an inputof the control signal instructing an increase in counting speed.Furthermore, the reception timestamp timer 209 is preset such that whenthe count value of the transmission timestamp timer 102 and the countvalue of the reception timestamp timer 209 are seen as the same time,the count value of the reception timestamp timer 209 is made smallerthan the count value of the reception timestamp timer 102 by an amountthat corresponds to a fixed delay time. Note that an exemplary structureof the reception timestamp timer 209 is described further below withreference to FIG. 5.

The comparison unit 210 compares the count value indicated by thetimestamp received from the timestamp buffer 204 and the count valuereceived from the reception timestamp timer 209, and if the receivedcount values match, the comparison unit 210 outputs a packet read signalfor instructing the packet read unit 211 to read a packet. Note thatsince the first reception buffer 203 and the timestamp buffer 204 areconstituted from FIFO buffers, the timestamp that is retrieved from thetimestamp buffer 204 is the timestamp that is attached to the packetthat is to be read from the first reception buffer 203.

The packet read unit 211 receives the packet read signal from thecomparison unit 210, and thereafter causes the first reception buffer203 to output one packet.

Reception Timestamp Timer

The following describes the structure of the reception timestamp timer209 included in the reception device 200 of FIG. 1, with reference toFIG. 5. FIG. 5 shows the structure of the reception timestamp timer 209shown in FIG. 1.

The reception timestamp timer 209 includes a variable frequencyoscillator 209 a, a counter 209 b, a control terminal 209 c, a countvalue output terminal 209 d, an initial value input terminal 209 e, anda clock output terminal 209 f.

The variable frequency oscillator 209 a can oscillate at frequencieswithin a predetermined range. The variable frequency oscillator 209 asupplies a clock signal at the oscillated frequency to the counter 209b, as well as externally outputs the clock signal via the clock outputterminal 209 f.

The variable frequency oscillator 209 a increases the oscillationfrequency upon receiving, from the control terminal 209 c, a controlsignal that is an instruction to increase the count speed, and decreasesthe oscillation frequency upon receiving a control signal that is aninstruction to decrease the count speed.

The counter 209 b retrieves, from the timestamp buffer 204 via the inputterminal 209 e, the timestamp that was stored first in the timestampbuffer 204, and is preset based on the count value indicated by theretrieved timestamp.

Note that since a packet received by the transmission device 100 isoutput from the reception device 200 at a delay equal to a transmissionfixed delay time, the counter 209 b must output a count value whilemaintaining an offset, with respect to the counter 102 b, thatcorresponds to the transmission fixed delay time. The counter 209 b istherefore preset such that when the count value of the counter 102 b andthe count value of the counter 209 b are seen as the same time, thecount value of the counter 209 b is made smaller than the count value ofthe counter 102 b by an amount that corresponds to the transmissionfixed delay time.

The counter 209 b counts the clock signal generated by the variablefrequency oscillator 209 a, and externally outputs the count value viathe count value output terminal 209 d. The count value output via thecount value output terminal 209 d is supplied to the comparison unit210.

Time-Base Recovery Processing

The counter 209 b that has been preset as described above counts theclock signal generated by the variable frequency oscillator 209 a, andoutputs a count value to the comparison unit 210 via the count valueoutput terminal 209 d.

The comparison unit 210 compares the count value indicated by thetimestamp retrieved from the timestamp buffer 204 and the count valuereceived from the reception timestamp timer 209, and if both countvalues match, the comparison unit 210 outputs the packet read signal tothe packet read unit 211. Upon receiving the packet read signal from thecomparison unit 210, the packet read unit 211 causes the first receptionbuffer 203 to output one packet.

According to this processing, as indicated by N105 in FIG. 3, thetime-base of the packet is recovered, and the packet is output from thereception device 200 at the transmission fixed delay time with respectto reception by the transmission device 100.

It should be noted that the output of packets from the reception device200 is controlled with use of a count value obtained by counting alimited frequency. For this reason, if the clock signal used forcounting in the transmission timestamp timer 102 and the receptiontimestamp timer 209 is asynchronous with the clock signal forprocessing, the packet will be output from the reception device 200 withresidual jitter that equals one or two clocks of the clock signal usedfor counting in the transmission timestamp timer 102 and the receptiontimestamp timer 209.

Rate Control Processing

The first reception buffer amount read unit 206 reads the firstreception buffer amount of the first reception buffer 203, and outputsthe read first reception buffer amount to the stored amount additionunit 207. The stored amount addition unit 207 retrieves the firsttransmission buffer amount from the first transmission buffer amountbuffer 205.

The stored amount addition unit 207 adds the first transmission bufferamount retrieved from the first transmission buffer amount buffer 205and the first reception buffer amount received from the first receptionbuffer amount read unit 206.

If the result of the addition increases, the correction unit 208 outputsa control signal for instructing the reception timestamp timer 209 toincrease the counting speed. Upon receiving this control signal, thereception timestamp timer 209 increases the counting speed.

If the result of the addition decreases, the correction unit 208 outputsa control signal for instructing the reception timestamp timer 209 todecrease the counting speed. Upon receiving this control signal, thereception timestamp timer 209 decreases the counting speed.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device evenif the input bit rate at which packets are input to the transmissiondevice is a variable bit rate and the transmission bit rate on thetransmission line is a constant bit rate.

Supplementary Remarks

(1) In the first embodiment and the hereinafter-described embodiment,the stored amounts of packets to which timestamps and transmissionbuffer amounts have not been attached are used as the stored amount ofpackets in the first reception buffer 203 and stored amount of packetsin the first transmission buffer 105 used in the correction of thecounting speed in the reception timestamp timer 209. However, thepresent invention is not limited to this. The stored amounts of packetsmay include either, or both, timestamps and transmission buffer amounts.

(2) In the first embodiment and the hereinafter-described embodiment,the frequency of the clock signal generated by the variable frequencyoscillator 209 a is controlled so as to enable changing the countingspeed of the reception timestamp timer 209. However, the counting speedof the reception timestamp timer 209 may be changed by making thefrequency of the clock signal a constant frequency and changing themethod by which the counter 209 b performs counting.

Embodiment 2

The following describes a synchronous transmission system pertaining toembodiment 2 of the present invention that realizes synchronoustransmission between a transmission device and a reception device, withreference to the drawings.

The present embodiment is the same as embodiment 1, with the addition ofa function for realizing synchronous transmission between transmissionand reception devices even if a packet is lost on the transmission line.

Note that the same reference characters have been given to constituentelements of embodiment 2 that are substantially the same as embodiment1, and descriptions thereof have been omitted since the descriptions inembodiment 1 are applicable.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 6. FIG. 6 showsthe structure of the synchronous transmission system of the presentembodiment.

A transmission device 100 a and a reception device 200 a transmit andreceive packets via the transmission line 300. Note that packets inputto the transmission device 100 a have a fixed length.

Transmission Device

The transmission device 100 a has the same structure as the transmissiondevice 100 of embodiment 1, with the addition of a sequence numbergeneration unit 121 and a sequence number attachment unit 122.

The sequence number generation unit 121 sequentially generates numbersbeginning from a value of “1” (hereinafter, called a “sequence number”),and supplies the generated sequence number to the sequence numberattachment unit 122.

The sequence number attachment unit 122 attaches the sequence numbersupplied from the sequence number generation unit 121 to the beginningof a joined packet output from the first transmission buffer 105.

In the transmission device 100 a, when a packet is input to the inputterminal 101, the timestamp attachment unit 103 attaches the count valuesupplied from the transmission timestamp timer 102 to the beginning ofthe input packet as a timestamp, and furthermore, the first transmissionbuffer amount attachment unit 104 attaches the first transmission bufferamount read by the first transmission buffer amount read unit 106 to thebeginning of the packet having the timestamp attached thereto. Thepacket having the timestamp and first transmission buffer amountattached thereto is stored in the first transmission buffer 105.

The first transmission buffer 105 outputs a joined packet composed of afixed number of packets that have been joined together. The sequencenumber attachment unit 122 attaches the sequence number supplied fromthe sequence number generation unit 121 to the beginning of the joinedpacket output from the first transmission buffer 105. The transmissionprocessing unit 107 performs predetermined processing on the joinedpacket having the sequence number attached thereto, and outputs theresulting packet to the transmission line.

Reception Device

The reception device 200 a has the same structure as the receptiondevice 200 of embodiment 1, with the addition of a missing packetdetection unit 221 and a buffer amount correction unit 222, and aseparation unit 202 a instead of the separation unit 202.

The separation unit 202 a receives the joined packet having the sequencenumber attached thereto from the reception processing unit 201, andseparates the received joined packet into the sequence number, aplurality of first transmission buffer amounts, a plurality oftimestamps, and a plurality of packets. The separation unit 202 aoutputs the sequence number, first transmission buffer amounts,timestamps, and packets to the missing packet detection unit 221, thefirst transmission buffer amount buffer 205, the timestamp buffer 204,and the first reception buffer 203 respectively.

The missing packet detection unit 221 detects, based on the sequencenumber received from the separation unit 202 a, the number of packetsthat were lost on the transmission line between the received joinedpacket and the joined packet that was received immediately therebefore,and outputs the detected number of missing packets to the buffer amountcorrection unit 222. Note that an exemplary structure of the missingpacket detection unit 221 is described later with reference to FIG. 7.

With reference to the number of missing packets received from themissing packet detection unit 221, the buffer amount correction unit 222stores “missing” in correspondence with a sequence number (to indicatethat the joined packet having that sequence number attached thereto wasnot received) or “received” in correspondence with a sequence number (toindicate that the joined packet having that sequence number attachedthereto was received).

For example, if 1, 0, 2, and 0 are received from the missing packetdetection unit 221 as the numbers of missing joined packets, the bufferamount correction unit 222 stores “missing” in correspondence withsequence number “1”, “received” in correspondence with sequence number“2”, “received in correspondence with sequence number “3”, “missing” incorrespondence with sequence number “4”, “missing” in correspondencewith sequence number “5”, “received” in correspondence with sequencenumber “6”, and “received” in correspondence with sequence number “7”.

The buffer amount correction unit 222 counts the number of packetsoutput from the first reception buffer 203, specifies which of thesequence numbers the packet that is to be output next corresponds to,and specifies, from among the sequence numbers including and after thespecified sequence number, the number of sequence numbers that arestored in correspondence with “missing”.

The buffer amount correction unit 222 multiplies the specified number ofsequence numbers by the number of packets included in the joined packet,and multiplies the result by the data size of the packet. The valueresulting from this multiplication is the data amount of the packetsthat were lost on the transmission line (hereinafter, called the missingamount) and that were supposed to be stored in the first receptionbuffer 203.

The buffer amount correction unit 222 adds the added value received fromthe stored amount addition unit 207 and the missing amount, and outputsthe resulting added value to the correction unit 208.

Instead of using the added value received from the stored amountaddition unit 207, the correction unit 208 of embodiment 2 uses theadded value received from the buffer amount correction unit 222 tocontrol the counting speed in the reception timestamp timer 209 so thatthe added value received from the buffer amount correction unit 222 is aconstant value.

If the added value increases, the counting speed of the receptiontimestamp timer 209 has become slower than the counting speed of thetransmission timestamp timer 102. The correction unit 208 thereforeoutputs a control signal instructing the reception timestamp timer 209to increase the counting speed. Upon receiving this control signal, thereception timestamp timer 209 increases the counting speed.

If the added value decreases, the counting speed of the receptiontimestamp timer 209 has become faster than the counting speed of thetransmission timestamp timer 102. The correction unit 208 thereforeoutputs a control signal instructing the reception timestamp timer 209to decrease the counting speed. Upon receiving this control signal, thereception timestamp timer 209 decreases the counting speed.

In the rate control processing of the present embodiment, the countingspeed of the reception timestamp timer 209 is adjusted so as to obtain aconstant value for the sum of the stored amount of packets in the firsttransmission buffer 105 (the first transmission buffer amount), thestored amount of packets in the first reception buffer 203 (the firstreception buffer amount), and the missing amount of packets that werelost on the transmission line and were supposed to be stored in thefirst reception buffer 203.

Note that a description of the time-base recovery processing performedby reception device 200 a has been omitted due to being substantiallythe same as the time-base recovery processing in embodiment 1.

Missing Packet Detection Unit

The following describes the structure of the missing packet detectionunit 221 included in the reception device 200 of FIG. 6, with referenceto FIG. 7. FIG. 7 shows the structure of the missing packet detectionunit shown in FIG. 6.

The missing packet detection unit 221 includes a storage unit 221 a, anaddition unit 221 b, a subtraction unit 221 c, an input terminal 221 d,and an output terminal 221 e.

The storage unit 221 a initially stores a value of 0.

Upon receiving a sequence number from the input terminal 221 d, thestorage unit 221 a stores the received sequence number, and outputs thesequence number stored immediately previously (the value “0” when asequence number is input for the first time) to the addition unit 221 b.

The addition unit 221 b adds 1 to the value received from the storageunit 221 a, and outputs the resulting added value to the subtractionunit 221 c.

The subtraction unit 221 c subtracts the received added value from thesequence number received from the input terminal 221 d, and outputs theresulting subtracted value to the buffer amount correction unit 222 viathe output terminal 221 e. The subtracted value obtained by thesubtraction unit 221 c indicates the number of joined packets that aremissing between the joined packet having attached thereto the sequencenumber currently received from the input terminal 221 d and the joinedpacket having attached thereto the sequence number that was receivedimmediately previously from the input terminal 221 d.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device whenthe input bit rate at which packets are input to the transmission deviceis a variable bit rate and the transmission bit rate on the transmissionline is a constant bit rate, and even if a packet is lost on thetransmission line.

Embodiment 3

The following describes a synchronous transmission system pertaining toembodiment 3 of the present invention that realizes synchronoustransmission between a transmission device and a reception device, withreference to the drawings.

Embodiments 1 and 2 are directed at a case in which the bit rate atwhich packets are input to the transmission device is a variable bitrate, and the transmission bit rate on the transmission line is aconstant bit rate. In contrast, the present embodiment and thelater-described embodiment 4 are directed at case in which the input bitrate at which packets are input to the transmission device is a constantbit rate, and the transmission bit rate on the transmission line is avariable bit rate.

Note that the same reference characters have been given to constituentelements of the present embodiment that have substantially the samefunctions as in embodiment 1, and descriptions thereof have been omittedsince the descriptions in embodiment 1 are applicable.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 8. FIG. 8 showsthe structure of the synchronous transmission system of the presentembodiment.

A transmission device 100 b and a reception device 200 b transmit andreceive packets via a transmission line 300 b. The transmission line 300b is, for example, a wireless transmission line.

Stored Amount of Packets

Prior to the description of the constituent elements shown in FIG. 8,the following describes the stored amount of packets in a secondtransmission buffer 105 b and a second reception buffer 203 b shown inFIG. 8, with reference to FIGS. 9A and 9B. FIGS. 9A and 9B show changesin bit rate and stored amount of packets over time in the synchronoustransmission system of FIG. 8.

The input bit rate at which packets are input to the second transmissionbuffer 105 b of the transmission device 100 b is a constant bit rate,and is expressed by a rate function f2(t). Also, the transmission bitrate on the transmission line is a variable bit rate, and is expressedby a rate function g2(t).

In synchronous transmission, the output bit rate at which packets areoutput from the second reception buffer 203 b is controlled so as to beequal to the input bit rate at which packets are input to the secondtransmission buffer 105 b. For this reason, letting “delay” express adelay time from when a packet is input to the transmission device 100 buntil when a packet is output from the reception device 200 b, the ratefunction f2(t-delay) expresses the output bit rate at which packets areoutput from the second reception buffer 203 b.

Here, (1) packet writing to the second transmission buffer 105 b isbegun at time 0, (2) packet reading from the second transmission buffer105 b and packet writing to the second reception buffer 203 b are begunat time t0, and (3) packet reading from the second reception buffer 203b is begun at time t1 (=delay).

In this case, the rate function f2(t) is c (c being a constant value),and the function g2(t) is 0 until time t0, and a variable value at andafter time t0 (FIG. 9A). Note that the function f2(t-delay) is 0 untiltime t1 (=delay), and c (c being a constant value) at and after time t1.

Letting BuffTx2(t) be a stored amount function expressing the storedamount of packets in the second transmission buffer 105 b at time t, thefollowing expression (6) indicates the stored amount function BuffTx2(t)at time t, which is at or after time t1 (FIG. 9B).

$\begin{matrix}{{{Expression}\mspace{14mu} (6)}\mspace{596mu}} & \; \\{{{BuffTx}\; 2(t)} = {{{\int_{0}^{t}{f\; 2(t){t}}} - {\int_{0}^{t}{g\; 2(t){t}}}} = {{\int_{0}^{t}{c{t}}} - {\int_{t\; 0}^{t}{f\; 2(t){t}}}}}} & (6)\end{matrix}$

Letting BuffRx2(t) be a stored amount function expressing the storedamount of packets in the second reception buffer 203 b at time t, thefollowing expression (7) indicates the stored amount function BuffRx2(t)at time t, which is at or after time t1 (FIG. 9B)

$\begin{matrix}{{{Expression}\mspace{14mu} (7)}\mspace{596mu}} & \; \\{{{BuffRx}\; 2(t)} = {{{\int_{0}^{t}{g\; 2(t){t}}} - {\int_{0}^{t}{f\; 2\left( {t - {delay}} \right){t}}}} = {{\int_{t\; 0}^{t}{g\; 2(t){t}}} - {\int_{t\; 0}^{t}{c{t}}}}}} & (7)\end{matrix}$

Based on expressions (6) and (7), the following expression (8) indicatesa sum of the stored amount of packets in the second transmission buffer105 b at time t, which is at or after time t1, and the stored amount ofpackets in the second reception buffer 203 b at time t.

$\begin{matrix}{{{Expression}\mspace{14mu} (8)}\mspace{596mu}} & \; \\{{{{BuffTx}\; 2(t)} + {{BuffRx}\; 2(t)}} = {{{\int_{0}^{t}{c{t}}} - {\int_{t\; 0}^{t}{g\; 2(t){t}}} + {\int_{t\; 0}^{t}{g\; 2(t){t}}} - {\int_{t\; 0}^{t}{c{t}}}} = {{\int_{0}^{t\; 0}{c{t}}} = {{constant}\mspace{14mu} {value}}}}} & (8)\end{matrix}$

As shown above, a constant value is obtained for the sum of the storedamount of packets in the second transmission buffer 105 b at time t,which is at or after time t1, and the stored amount of packets in thesecond reception buffer 203 b at time t (FIG. 9B).

In other words, a constant value is obtained for the sum of the storedamount of packets in the second transmission buffer 105 b just after acertain packet has been read from the second transmission buffer 105 band the stored amount of packets in the second reception buffer 203 bjust after the certain packet has been written to the second receptionbuffer 203 b. Also, a constant value is obtained for the sum of thestored amount of packets in the second transmission buffer 105 b justbefore a certain packet is read from the second transmission buffer 105b and the stored amount of packets in the second reception buffer 203 bjust before the certain packet is written to the second reception buffer203 b.

In the present embodiment, synchronous transmission between thetransmission device 100 b and reception device 200 b is realized byutilizing the relationship in which the sum of the stored packet amountsis constant.

The present embodiment makes use of the sum of the stored amount ofpackets in the second transmission buffer 105 b just after a certainpacket has been read from the second transmission buffer 105 b and thestored amount of packets in the second reception buffer 203 b just afterthe certain packet has been written to the second reception buffer 203b.

Note that the sum of the stored amount of packets in the secondtransmission buffer 105 b just before a certain packet is read from thesecond transmission buffer 105 b and the stored amount of packets in thesecond reception buffer 203 b just before the certain packet is writtento the second reception buffer 203 b may be used.

Although it is desirable to use either of the above sums to realizesynchronous transmission, when a sufficient amount of packets is storedin the second transmission buffer 105 b and the second reception buffer203 b, there may be a slight difference in the timing at which thestored amounts of the second transmission buffer 105 b and the secondreception buffer 203 b are acquired.

Transmission Device

The transmission device 100 b includes the input terminal 101, thetransmission timestamp timer 102, the timestamp attachment unit 103, thesecond transmission buffer 105 b, a second transmission buffer amountread unit 106 b, a second transmission buffer amount attachment unit 104b, and the transmission processing unit 107.

The second transmission buffer 105 b is constituted from a FIFO buffer.The second transmission buffer 105 b temporarily stores input packets,and outputs the stored packets at a variable bit rate. Note that thesecond transmission buffer 105 b joins a plurality of packets togetherand outputs the group of packets (the joined packet) in order toincrease transmission efficiency.

The second transmission buffer amount read unit 106 b reads the storedamount of packets in the second transmission buffer 105 b (hereinafter,called the “second transmission buffer amount”), and outputs the secondtransmission buffer amount to the second transmission buffer amountattachment unit 104 b. Here, the second transmission buffer amount isthe stored amount of packets in the second transmission buffer 105 bjust after the reading, from the second transmission buffer 105 b, ofthe packet to which the second transmission buffer amount is to beattached by the second transmission buffer amount attachment unit 104 b.

In order to facilitate processing in the reception device 200, thesecond transmission buffer amount is the stored amount of data in thesecond transmission buffer 105 b excluding the data amount of thetimestamp.

The second transmission buffer amount can be calculated in, for example,the following way. The data size of the timestamp is assumed to be afixed data size. The data size of the timestamp is multiplied by thenumber of packets stored in the second transmission buffer 105 b (avalue equaling the number of input packets minus the number of outputpackets). The value resulting from the multiplication is subtracted fromthe actual stored amount of the second transmission buffer 105 b. Thevalue resulting from the subtraction is the second transmission bufferamount.

The second transmission buffer amount attachment unit 104 b attaches thesecond transmission buffer amount, which is input from the secondtransmission buffer amount read unit 106 b, to the beginning of thejoined packet received from the second transmission buffer 105 b.

In the transmission device 100 b, when a packet is input to the inputterminal 101 (N301 of FIG. 10), the timestamp attachment unit 103attaches a count value supplied from the transmission timestamp timer102 to the beginning of the input packet as a timestamp (N302). Thepacket having the timestamp attached thereto is stored in the secondtransmission buffer 105 b.

The second transmission buffer 105 b outputs the joined packet (N303).The second transmission buffer amount attachment unit 104 b attaches asecond transmission buffer amount read by the second transmission bufferamount read unit 106 b to the beginning of the joined packet (N304). Thetransmission processing unit 107 performs predetermined processing onthe joined packet having the second transmission buffer amount attachedthereto, and the resulting packet is sent to the transmission line 300b.

Reception Device

The reception device 200 b performs time-base recovery processing, ratecontrol processing and the like, which are conditions of synchronoustransmission. The reception device 200 b includes the receptionprocessing unit 201, a separation unit 202 b, the second receptionbuffer 203 b, the timestamp buffer 204, a second reception buffer amountread unit 206 b, a stored amount addition unit 207 b, a correction unit208 b, the reception timestamp timer 209, the comparison unit 210, thepacket read unit 211, and the output terminal 212.

The separation unit 202 b receives, from the reception processing unit201, a joined packet having a second transmission buffer amount attachedthereto, and separates the received joined packet into a secondtransmission buffer amount, a plurality of timestamps, and a pluralityof packets. The separation unit 202 b outputs the second transmissionbuffer amount, timestamps, and packets to the stored amount additionunit 207 b, timestamp buffer 204, and second reception buffer 203 brespectively.

The second reception buffer 203 b is constituted from a FIFO buffer, andtemporarily stores input packets. Packets output from the secondreception buffer 203 b are externally sent via the output terminal 212(N305 of FIG. 10).

The second reception buffer amount read unit 206 b reads the storedamount of packets in the second reception buffer 203 b (hereinafter,called the “second reception buffer amount”), and outputs the readsecond reception buffer amount to the stored amount addition unit 207 b.The second reception buffer amount is the stored amount of packets inthe second reception buffer 203 b just after the writing to the secondreception buffer 203 b of the packets constituting the joined packethaving attached thereto the second transmission buffer amount that wasoutput to the stored amount addition unit 207 b.

The stored amount addition unit 207 b adds the second transmissionbuffer amount that was received from the separation unit 202 b and thesecond reception buffer amount that was received from the secondreception buffer amount read unit 206 b.

Note that just after the packets included in the joined packet inputfrom the separation unit 202 b have been written to the second receptionbuffer 203 b, the second transmission buffer amount that is input to thestored amount addition unit 207 b by the separation unit 202 b is thesecond transmission buffer amount that was attached to the joined packetwhose packets were just previously written to the second receptionbuffer 203 b.

The correction unit 208 b controls the counting speed of the receptiontimestamp timer 209 so that the added value received as input from thestored amount addition unit 207 b is constant.

If the added value increases, the counting speed of the receptiontimestamp timer 209 has become slower than the counting speed of thetransmission timestamp timer 102. The correction unit 208 b thereforeoutputs a control signal instructing the reception timestamp timer 209to increase the counting speed.

If the added value decreases, the counting speed of the receptiontimestamp timer 209 has become faster than the counting speed of thetransmission timestamp timer 102. The correction unit 208 b thereforeoutputs a control signal instructing the reception timestamp timer 209to decrease the counting speed.

Note that a description of the time-base recovery processing performedby reception device 200 b has been omitted due to being substantiallythe same as the time-base recovery processing in embodiment 1.

Rate Control Processing

The second reception buffer amount read unit 206 b reads the secondreception buffer amount of the second reception buffer 203 b, andoutputs the read second reception buffer amount to the stored amountaddition unit 207 b. The stored amount addition unit 207 b receives aninput of the second transmission buffer amount from the separation unit202 b.

The stored amount addition unit 207 b adds the second transmissionbuffer amount received from the separation unit 202 b and the secondreception buffer amount received from the second reception buffer amountread unit 206.

If the result of the addition increases, the correction unit 208 boutputs a control signal for instructing the reception timestamp timer209 to increase the counting speed. Upon receiving this control signal,the reception timestamp timer 209 increases the counting speed.

If the result of the addition decreases, the correction unit 208 boutputs a control signal for instructing the reception timestamp timer209 to decrease the counting speed. Upon receiving this control signal,the reception timestamp timer 209 decreases the counting speed.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device evenif the input bit rate at which packets are input to the transmissiondevice is a constant bit rate and the transmission bit rate on thetransmission line is a variable bit rate.

Supplementary Remarks

(1) In the third embodiment and the hereinafter-described embodiment,the stored amounts of packets to which timestamps have not been attachedare used as the stored amount of packets in the second transmissionbuffer 105 b and stored amount of packets in the second reception buffer203 b used in the correction of the counting speed in the receptiontimestamp timer 209. However, the present invention is not limited tothis. The stored amounts of packets may include the timestamps.

Embodiment 4

The following describes a synchronous transmission system pertaining toembodiment 4 of the present invention that realizes synchronoustransmission between a transmission device and a reception device, withreference to the drawings.

The present embodiment is the same as embodiment 3, with the addition ofa function for realizing synchronous transmission between transmissionand reception devices even if a packet is lost on the transmission line.

Note that the same reference characters have been given to constituentelements of embodiment 4 that are substantially the same as embodiments1 to 3, and descriptions thereof have been omitted since thedescriptions in embodiments 1 to 3 are applicable.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 11. FIG. 11shows the structure of the synchronous transmission system of thepresent embodiment.

A transmission device 100 c and a reception device 200 c transmit andreceive packets via the transmission line 300 b. Note that packets inputto the transmission device 100 c have a fixed length.

Transmission Device

The transmission device 100 c has the same structure as the transmissiondevice 100 b of embodiment 3, with the addition of a sequence numbergeneration unit 121 and a sequence number attachment unit 122.

In the transmission device 100 c, when a packet is input to the inputterminal 101, the timestamp attachment unit 103 attaches the count valuesupplied from the transmission timestamp timer 102 to the beginning ofthe input packet as a timestamp. The packet having the timestampattached thereto is stored in the second transmission buffer 105 b.

The second transmission buffer 105 b outputs a joined packet composed ofa fixed number of packets that have been joined together. The secondtransmission buffer amount attachment unit 104 b attaches the secondtransmission buffer amount read by the second transmission buffer amountread unit 106 b to the beginning of the joined packet, and the sequencenumber attachment unit 122 attaches the sequence number supplied fromthe sequence number generation unit 121 to the beginning of the joinedpacket having the second transmission buffer amount attached thereto.The transmission processing unit 107 performs predetermined processingon the joined packet having the sequence number and second transmissionbuffer amount attached thereto, and outputs the resulting packet to thetransmission line 300 b.

Reception Device

The reception device 200 c has the same structure as the receptiondevice 200 b of embodiment 3, with the addition of a missing packetdetection unit 221 and a buffer amount correction unit 222, and aseparation unit 202 c instead of the separation unit 202 b.

The separation unit 202 c receives the joined packet having the sequencenumber and second transmission buffer amount attached thereto from thereception processing unit 201, and separates the received joined packetinto the sequence number, the second transmission buffer amount, aplurality of timestamps, and a plurality of packets. The separation unit202 c outputs the sequence number, second transmission buffer amount,timestamps, and packets to the missing packet detection unit 221, thestored amount addition unit 207 b, the timestamp buffer 204, and thesecond reception buffer 203 b respectively.

The reception device 200 c performs rate control processing as follows.

Just after the packets included in the joined packet input from theseparation unit 202 c have been written to the second reception buffer203 b, the second reception buffer amount read unit 206 b reads thesecond reception buffer amount of the second reception buffer 203 b, andoutputs the read second reception buffer amount to the stored amountaddition unit 207 b.

Also, in the same way as described in embodiment 2, the missing packetdetection unit 221 detects, based on the sequence number received fromthe separation unit 202 c, the number of packets that were lost on thetransmission line, and outputs the detected number of missing packets tothe buffer amount correction unit 222.

The stored amount addition unit 207 b adds the second transmissionbuffer amount received from the separation unit 202 c and the secondreception buffer amount received from the second reception buffer amountread unit 206 b.

In the same way as described in embodiment 2, the buffer amountcorrection unit 222 calculates the data amount of packets included in ajoined packet that was lost on the transmission line (missing packetamount) and that were supposed to be stored in the second receptionbuffer 203 b. The buffer amount correction unit 222 then adds thecalculated missing packet amount to the added value received from thestored amount addition unit 207 b.

Based on the added value received from the buffer amount correction unit222, the correction unit 208 b controls the counting speed of thereception timestamp timer 209 so that the added value received from thebuffer amount correction unit 222 is constant.

If the added value increases, the counting speed of the receptiontimestamp timer 209 has become slower than the counting speed of thetransmission timestamp timer 102. The correction unit 208 b thereforeoutputs a control signal instructing the reception timestamp timer 209to increase the counting speed. Upon receiving this control signal, thereception timestamp timer 209 increases the counting speed.

If the added value decreases, the counting speed of the receptiontimestamp timer 209 has become faster than the counting speed of thetransmission timestamp timer 102. The correction unit 208 b thereforeoutputs a control signal instructing the reception timestamp timer 209to decrease the counting speed. Upon receiving this control signal, thereception timestamp timer 209 decreases the counting speed.

Note that a description of the time-base recovery processing performedby reception device 200 c has been omitted due to being substantiallythe same as the time-base recovery processing in embodiment 1.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device whenthe input bit rate at which packets are input to the transmission deviceis a constant bit rate and the transmission bit rate on the transmissionline is a variable bit rate, and even if a packet is lost on thetransmission line.

Embodiment 5

The following describes a synchronous transmission system pertaining toembodiment 5 of the present invention that realizes synchronoustransmission between a transmission device and a reception device, withreference to the drawings.

Embodiments 1 and 2 are directed at a case in which the bit rate atwhich packets are input to the transmission device is a variable bitrate, and the transmission bit rate on the transmission line is aconstant bit rate. Embodiments 3 and 4 are directed at case in which thebit rate at which packets are input to the transmission device is aconstant bit rate, and the transmission bit rate on the transmissionline is a variable bit rate. In contrast, the present embodiment and thelater-described embodiments 6 and 7 are directed at a case in which thebit rate at which packets are input to the transmission device is avariable bit rate, and the transmission bit rate on the transmissionline is a variable bit rate.

Note that the same reference characters have been given to constituentelements of the present embodiment that have substantially the samefunctions as in embodiments 1 to 4, and descriptions thereof have beenomitted since the descriptions in embodiments 1 to 4 are applicable.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 12. FIG. 12shows the structure of the synchronous transmission system of thepresent embodiment.

A transmission device 100 d and a reception device 200 d transmit andreceive packets via the transmission line 300 b.

Transmission Device

The transmission device 100 d includes the input terminal 101, a firsttransmission unit 150, and a second transmission unit 160.

The first transmission unit 150 has the same structure as thetransmission device 100 of embodiment 1, with the exception of lackingthe input terminal 101 and the transmission processing unit 107. Thefirst transmission unit 150 outputs packets that were received at avariable input rate, to the second transmission unit 160 at a constantbit rate. Control for outputting packets from the first transmissionbuffer 105 at a constant bit rate is realized by, in the exemplary caseof an MPEG2 TS, extracting the stream rate from the stream header, andoutputting the packets at a stream rate obtained by adding the extractedstream rate and the rate corresponding to the timestamps attached to theinput packets and the rate corresponding to the overhead data of thefirst transmission buffer amount.

The second transmission unit 160 has the same structure as thetransmission device 100 b of embodiment 3, with the exception of lackingthe input terminal 101, the transmission timestamp timer 102, and thetimestamp attachment unit 103. The second transmission unit 160 outputspackets received from the first transmission unit 150 at a constant bitrate, to the transmission line 300 b at a variable bit rate.

In the transmission device 100 d, when a packet is input to the inputterminal 101 (N501 in FIG. 13), the timestamp attachment unit 103attaches the count value supplied from the transmission timestamp timer102 to the beginning of the input packet as a timestamp (N502), andfurthermore, the first transmission buffer amount attachment unit 104attaches the first transmission buffer amount read by the firsttransmission buffer amount read unit 106 to the beginning of the packethaving the timestamp attached thereto (N503). The packet having thetimestamp and first transmission buffer amount attached thereto isstored in the first transmission buffer 105.

The packet that is stored in the first transmission buffer 105 is outputat a constant bit rate, and stored in a second transmission buffer 105b.

The second transmission buffer 105 b outputs a joined packet composed ofa fixed number of packets that have been joined together (N504). Thesecond transmission buffer amount attachment unit 104 b attaches asecond transmission buffer amount read by the second transmission bufferamount read unit 106 b to the beginning of the joined packet (N505). Thetransmission processing unit 107 performs predetermined processing onthe joined packet having the second transmission buffer amount attachedthereto, and the resulting packet is sent to the transmission line 300b.

Reception Device

The reception device 200 d includes a second reception unit 260, a firstreception unit 250, and the output terminal 212.

The second reception unit 260 performs substantially the same ratecontrol processing as the rate control processing performed by thereception device 200 b. However, the second reception unit 260 does notperform the time-base recovery processing performed by the receptiondevice 200 b. This is because it is sufficient for time-base recovery tobe performed when packets are output from the reception device 200 d,and it is sufficient for the second reception unit 260 to output packetsto the first reception unit 250 at a constant bit rate that is equal tothe constant bit rate at which packets are output from the firsttransmission unit 150 to the second transmission unit 160.

The second reception unit 260 includes the reception processing unit201, a separation unit 202 d, the second transmission buffer 203 b, thesecond reception buffer amount read unit 206 b, the stored amountaddition unit 207 b, the correction unit 208 b, and a read rate controlunit 251.

The separation unit 202 d receives, from the reception processing unit201, a joined packet having a second transmission buffer amount attachedthereto, and separates the received joined packet into a plurality offirst transmission buffer amounts, a second transmission buffer amount,a plurality of timestamps, and a plurality of packets. The separationunit 202 b outputs the first transmission buffer amounts, the secondtransmission buffer amount, timestamps, and packets to the firsttransmission buffer amount buffer 205, the stored amount addition unit207 b, timestamp buffer 204, and second reception buffer 203 brespectively.

Instead of controlling the oscillation frequency of the variablefrequency oscillator 209 a of the reception timestamp timer 209, thecorrection unit 208 b of the present embodiment performs later-describedcontrol of the oscillation frequency by a later-described variablefrequency oscillator 251 a included in the read rate control unit 251.

The read rate control unit 251 controls the output bit rate at whichpackets are output from the second reception buffer 203 b.

The second reception unit 260 performs rate control processing asfollows.

The stored amount of packets in the second transmission buffer 105 bjust after the joined packet has been read from the second transmissionbuffer 105 b (the second transmission buffer amount received from theseparation unit 202 d) is added to the stored amount of packets in thesecond reception buffer 203 b just after the plurality of packetsincluded in the joined packet have been written to the second receptionbuffer 203 b (the second reception buffer amount that is read by andreceived from the second reception buffer amount read unit 206 b) toobtain an added value, and the correction unit 208 b controls the readrate control unit 251 so that the added value is a constant value.

If the added value increases, the bit rate at which packets are outputfrom the second reception buffer 203 b has become lower than the bitrate at which packets are output from the second transmission buffer 105b. The correction unit 208 b therefore outputs a control signalinstructing the read rate control unit 251 to increase the output bitrate.

If the added value decreases, the bit rate at which packets are outputfrom the second reception buffer 203 b has become higher than the bitrate at which packets are output from the second transmission buffer 105b. The correction unit 208 b therefore outputs a control signalinstructing the read rate control unit 251 to decrease the output bitrate.

Based on the control signal received from the correction unit 208 b, theread rate control unit 251 controls the bit rate at which packets areoutput from the second reception buffer 203 b.

Read Rate Control Unit

The following describes the structure of the read rate control unit 251included in the reception device 200 d of FIG. 12, with reference toFIG. 14. FIG. 14 shows the structure of the read rate control unit 251shown in FIG. 12.

The read rate control unit 251 includes the variable frequencyoscillator 251 a, a CBR read unit 251 b, a control signal input terminal251 c, a read signal output terminal 251 d, and a clock output terminal251 e.

The variable frequency oscillator 251 a can oscillate at frequencieswithin a predetermined range. The variable frequency oscillator 251 asupplies a clock signal at the oscillated frequency to the counter 209b, as well as outputs the clock signal to the clock output terminal 251e.

The variable frequency oscillator 251 a increases the oscillationfrequency upon receiving, from the control terminal 209 c, a controlsignal that is an instruction to increase the bit rate at which packetsare read from the second reception buffer 203 b, and decreases theoscillation frequency upon receiving a control signal that is aninstruction to decrease the bit rate at which packets are read from thesecond reception buffer 203 b.

The CBR read unit 251 b includes a counter that counts the clock signalreceived from the variable frequency oscillator 251 a, and generates aread signal upon counting a certain number of clock signals. The CBRread unit 251 b outputs the generated read signal to the secondreception buffer 203 b via the read signal output terminal 251 d. Uponreceiving the read signal, the second reception buffer 203 b outputs onestored packet to the first reception buffer 203.

Returning to the description of FIG. 12, the first reception unit 250has the same structure as the reception device 200 of embodiment 1, withthe exception of lacking the reception processing unit 201, theseparation unit 202, and the output terminal 212. The first receptionunit 250 performs substantially the same time-base recovery processingand rate control processing as the reception device 200 of embodiment 1.

The first reception unit 250 performs rate control processing asfollows.

The stored amount of packets in the first transmission buffer 105 justbefore a certain packet is written to the first transmission buffer 105(the first transmission buffer amount that was retrieved from the firsttransmission buffer amount buffer 205) is added to the stored amount ofpackets in the first reception buffer 203 just before the certain packetis read from the first reception buffer (the first reception bufferamount that is read by and received from the first reception bufferamount read unit 206) to obtain an added value, and the correction unit208 controls the oscillation frequency of the variable frequencyoscillator 209 a in the reception timestamp timer 209 so that the addedvalue is a constant value.

If the added value increases, the correction unit 208 controls thevariable frequency oscillator 209 a to increase the oscillationfrequency, and if the added value decreases, the correction unit 208controls the variable frequency oscillator 209 a to decrease theoscillation frequency.

The first reception unit 250 performs time-base recovery processing asfollows.

The variable frequency oscillator 209 a in the reception timestamp timer209 oscillates at a frequency according to the control of the correctionunit 208, and the counter 209 b counts the clock signal generated by thevariable frequency oscillator 209 a.

The comparison unit 210 compares the count value indicated by thetimestamp retrieved from the timestamp buffer 204 and the count valuereceived from the counter 209 b, and if both count values match, thecomparison unit 210 outputs the packet read signal to the packet readunit 211. Upon receiving the packet read signal from the comparison unit210, the packet read unit 211 causes the first reception buffer 203 tooutput one packet. Accordingly, one packet is externally output from thefirst reception buffer 203 via the output terminal 212.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device evenif the input bit rate at which packets are input to the transmissiondevice is a variable bit rate and the transmission bit rate on thetransmission line is a variable bit rate.

Embodiment 6

The following describes a synchronous transmission system pertaining toembodiment 6 of the present invention that realizes synchronoustransmission between a transmission device and a reception device, withreference to the drawings.

Embodiment 6 describes a simplified structure of the transmission device100 d and reception device 200 d of embodiment 5, and enablessynchronous transmission even if both the input bit rate andtransmission bit rate of packets are variable bit rates.

Note that the same reference characters have been given to constituentelements of the present embodiment that have substantially the samefunctions as in embodiments 1 to 5, and descriptions thereof have beenomitted since the descriptions in embodiments 1 to 5 are applicable.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 15. FIG. 15shows the structure of the synchronous transmission system of thepresent embodiment.

A transmission device 100 e and a reception device 200 e transmit andreceive packets via the transmission line 300 b.

Transmission Device

The transmission device 100 e includes the input terminal 101, a firsttransmission unit 150 e, and a second transmission unit 160 e that hasthe same structure as the second transmission unit 160 of embodiment 5.

The first transmission unit 150 e includes the transmission timestamptimer 102, the timestamp attachment unit 103, and the first transmissionbuffer 105 that temporarily stores packets input at a variable bit rateand outputs the stored packets at a constant bit rate. Control foroutputting packets from the first transmission buffer 105 at a constantbit rate is realized by, in the exemplary case of an MPEG2 TS,extracting the stream rate from the stream header, and outputting thepackets at a stream rate obtained by adding the extracted stream rateand the rate corresponding to the timestamps attached to the inputpackets and the rate corresponding to the overhead data of the firsttransmission buffer amount.

In the transmission device 100 e, when a packet is input to the inputterminal 101, the timestamp attachment unit 103 attaches a count valuesupplied from the transmission timestamp timer 102 to the beginning ofthe input packet as a timestamp. The packet having the timestampattached thereto is stored in the first transmission buffer 105.

The packet stored in the first transmission buffer 105 is output to thesecond transmission buffer 105 b at a constant bit rate, and stored inthe second transmission buffer 105 b.

The second transmission buffer 105 b outputs a joined packet composed ofa plurality of packets that have been joined together. The secondtransmission buffer amount attachment unit 104 b attaches the secondtransmission buffer amount read by the second transmission buffer amountread unit 106 b to the beginning of the joined packet. The transmissionprocessing unit 107 performs predetermined processing on the joinedpacket having the second transmission buffer amount attached thereto,and outputs the resulting packet to the transmission line 300 b.

Reception Device

The reception device 200 e includes a second reception unit 260 e, afirst reception unit 250 e, and an input terminal 221.

Similarly to the second reception unit 260 of embodiment 5, the secondreception unit 260 e performs substantially the same rate controlprocessing as the rate control processing performed by the receptiondevice 200 b, but does not perform the time-base recovery processingperformed by the reception device 200 b.

The second reception unit 260 e has the same structure as the secondreception unit 260 of embodiment 5, with the exception of a separationunit 202 e instead of the separation unit 202 d.

The separation unit 202 e receives a joined packet from the receptionprocessing unit 201, and separates the received joined packet into thesecond transmission buffer amount, a plurality of timestamps, and aplurality of packets. The separation unit 202 b outputs the secondtransmission buffer amount, timestamps, and packets to the stored amountaddition unit 207 b, timestamp buffer 204, and second reception buffer203 b respectively.

Note that a description of the rate control processing performed by thesecond reception unit 260 e has been omitted due to being substantiallythe same as the rate control processing performed by the secondreception unit 260 of embodiment 5.

It should be noted that in embodiment 6, the clock signal whosefrequency is controlled by the correction unit 208 b is output from theread rate control unit 251 to the reception timestamp timer 209 e viathe clock output terminal 251 e.

The first reception unit 250 e performs substantially the same time-baserecovery processing as the time-base recovery processing performed bythe reception device 200. However, the first reception unit 250 e doesnot perform the rate control processing performed by the receptiondevice 200 b.

The first reception unit 250 e does not perform rate control processingfor the following reason.

The second reception unit 260 e performs rate control processing. Thefrequency of the clock signal output by the read rate control unit 251of the second reception unit 260 is therefore adjusted to the samefrequency as the clock signal generated by the oscillator 102 a of thetransmission timestamp timer 102. Using the clock signal output by theread rate control unit 251 therefore eliminates the need to generate aclock signal whose frequency has been adjusted to be the same as theclock signal generated by the oscillator 102 a of the transmissiontimestamp timer 102.

The first reception unit 250 e includes a reception timestamp timer 209e, the comparison unit 210, the packet read unit 211, and the firstreception buffer 203.

The reception timestamp timer 209 e includes a counter that counts theclock signal received from the read rate control unit 251, and outputs acount value to the comparison unit 210. The counter included in thereception timestamp timer 209 e is preset in the same procedure as thecounter 209 b of the reception timestamp timer 209. Note that thereception timestamp timer 209 e does not include a variable frequencyoscillator like the reception timestamp timer 209.

The first reception unit 250 e performs time-base recovery as follows.

The preset reception timestamp timer 209 e counts the clock signalreceived from the read rate control unit 251, and outputs the countvalue to the comparison unit 210. The comparison unit 210 compares thecount value indicated by the timestamp retrieved from the timestampbuffer 204 and the count value received from the reception timestamptimer 209 e, and if both count values match, the comparison unit 210outputs the packet read signal to the packet read unit 211. Uponreceiving the packet read signal from the comparison unit 210, thepacket read unit 211 causes the first reception buffer 203 to output onepacket.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device thathave a simpler structure than embodiment 5, even if the input bit rateat which packets are input to the transmission device is a variable bitrate and the transmission bit rate on the transmission line is avariable bit rate.

Supplementary Remarks

(1) In embodiment 6, the following may be performed if the frequency ofthe oscillator 102 a in the transmission timestamp timer 102 isdifferent from the frequency of the variable frequency oscillator 251 ain the read rate control unit 251. A PLL circuit or the like may be usedto multiply or divide the frequency of the variable frequency oscillator251 a, and a clock signal having the multiplied or divided frequency maybe supplied to the reception timestamp timer 209 e.

Embodiment 7

The following describes a synchronous transmission system pertaining toembodiment 7 of the present invention that realizes synchronoustransmission between a transmission device and a reception device, withreference to the drawings.

Embodiment 7 describes a simplified structure of the transmission device100 d and reception device 200 d of embodiment 5, and enablessynchronous transmission even if both the input bit rate andtransmission bit rate of packets are variable bit rates.

Note that the same reference characters have been given to constituentelements of the present embodiment that have substantially the samefunctions as in embodiments 1 to 6, and descriptions thereof have beenomitted since the descriptions in embodiments 1 to 6 are applicable.

Synchronous Transmission System

The following describes the structure of the synchronous transmissionsystem of the present embodiment with reference to FIG. 16. FIG. 16shows the structure of the synchronous transmission system of thepresent embodiment.

A transmission device 100 f and a reception device 200 f transmit andreceive packets via the transmission line 300 b.

Transmission Device

The transmission device 100 f includes the input terminal 101, a firsttransmission unit 150 f that has the same structure as the firsttransmission unit 150 e of embodiment 5, and a second transmission unit160 f.

The second transmission unit 160 f includes the second transmissionbuffer 105 b and the transmission processing unit 107.

In the transmission device 100 e, when a packet is input to the inputterminal 101, the timestamp attachment unit 103 attaches a count valuesupplied from the transmission timestamp timer 102 to the beginning ofthe input packet as a timestamp, and furthermore, the first transmissionbuffer amount attachment unit 104 attaches a first transmission bufferamount read by the first transmission buffer amount read unit 106 to thebeginning of the packet having the timestamp attached thereto. Thepacket having the timestamp and first transmission buffer amountattached thereto is stored in the first transmission buffer 105.

The packet that is stored in the first transmission buffer 105 is outputat a constant bit rate, and stored in the second transmission buffer 105b. Control for outputting packets from the first transmission buffer 105at a constant bit rate is realized by, in the exemplary case of an MPEG2TS, extracting the stream rate from the stream header, and outputtingthe packets at a stream rate obtained by adding the extracted streamrate and the rate corresponding to the timestamps attached to the inputpackets and the rate corresponding to the overhead data of the firsttransmission buffer amount.

The second transmission buffer 105 b outputs a joined packet composed ofa plurality of packets that have been joined together, and thetransmission processing unit 107 performs predetermined processing onthe joined packet and outputs the resulting packet to the transmissionline 300 b.

Reception Device

The reception device 200 f includes a second reception unit 260 f, afirst reception unit 250 f, and the input terminal 221.

The second reception unit 260 f includes the reception processing unit201, a separation unit 202 f, a CBR read unit 271, and the secondreception buffer 203 b.

The separation unit 202 f receives a joined packet from the receptionprocessing unit 201, and separates the received joined packet into aplurality of first transmission buffer amounts, a plurality oftimestamps, and a plurality of packets. The separation unit 202 foutputs the first transmission buffer amounts, timestamps, and packetsto the first transmission buffer amount buffer 205, timestamp buffer204, and second reception buffer 203 b respectively.

The CBR read unit 271 includes a counter that counts the clock signalsupplied from the reception timestamp timer 209, and generates a readsignal upon counting a certain number of clock signals. Upon receivingthe read signal, the second reception buffer 203 b outputs one storedpacket to the first reception buffer 203.

The frequency of the clock signal input to the CBR read unit 271 fromthe reception timestamp timer 209 has been adjusted to be the same asthe frequency of the oscillator 102 a in the transmission timestamptimer 102. As such, in the present embodiment there is no need toperform rate control processing with use of the stored amount of packetsin the second transmission buffer 105 b and the stored amount of packetsin the second reception buffer 203 b.

Note that the second reception unit 260 f does not perform time-baserecovery processing because it is sufficient for time-base recovery tobe performed on packets that are output from the first reception buffer203.

The first reception unit 250 f has the same structure as the firstreception unit 250 of embodiment 5, and performs time-base recoveryprocessing and rate control processing. The clock signal generated bythe variable frequency oscillator 209 in the reception timestamp timer209 is output to the CBR read unit 271 via the clock output terminal 209f.

A description of the time-base recovery processing and rate controlprocessing performed by the first reception unit 250 f has been omittedsince they are the same as the time-base recovery processing and ratecontrol processing performed by the first reception unit 250 ofembodiment 5.

The present embodiment as described above enables realizing synchronoustransmission between a transmission device and a reception device thathave a simpler structure than embodiment 5, even if the input bit rateat which packets are input to the transmission device is a variable bitrate and the transmission bit rate on the transmission line is avariable bit rate.

Supplementary Remarks

(1) In the above-described embodiments, the input packets may be packetsrelating to an MPEG2 TS.

(2) In the above-described embodiments, the stored amount of packets inthe transmission buffer (the first transmission buffer 105 or secondtransmission buffer) is transmitted from the transmission device to thereception device, and is used in adjusting the counting speed of thecounter included in the reception device. However, the present inventionis not limited to this. The information that is transmitted from thetransmission device to the reception device may be any information thatenables specifying the stored amount of packets in the transmissionbuffer.

For example, the stored amount of packets in the transmission buffer isequivalent with a value obtained by subtracting the time integrationvalue of the rate at which packets are output from the transmissionbuffer from the time integration value of the rate at which packets areinput to the transmission buffer.

Similarly, the stored amount of packets in the reception buffer (thefirst reception buffer 203 or the second reception buffer 203 b) isequivalent with a value obtained by subtracting the time integrationvalue of the rate at which packets are output from the reception bufferfrom the time integration value of the rate at which packets are inputto the reception buffer.

Therefore, the values equal to the stored amounts of packets in thetransmission buffer and reception buffer may be calculated with use ofinput/output rates, and the counting speed of the counter may beadjusted so that the sum of the calculated values is constant.

In view of this, instead of transmitting the stored amount of packets inthe transmission buffer to the reception device, the transmission devicemay measure the input and output rates with respect to the transmissionbuffer, and transmit the measured input and output rates to thereception device. In such a case, the reception device measures theinput and output rates with respect to the reception buffer.

Note that since the time integration value of the rate of output fromthe transmission buffer is the same as the time integration value of therate of input to the reception buffer, these two time integration valuescancel each other out when adding the values equal to the stored amountsof packets in the transmission buffer and reception buffer. For thisreason, it is sufficient for only the rate of input to the transmissionbuffer to be transmitted to the reception device, and for the receptiondevice to only measure the rate of output from the reception buffer.

(3) The present invention may be reception methods for performingprocessing equivalent to the functions of part or all of the constituentelements in the above-described embodiments.

Alternatively, the present invention may be programs that describeprocessing equivalent to the functions of part or all of the constituentelements in the above-described embodiments, where the programs arestored in memory and executed by a CPU etc.

INDUSTRIAL APPLICABILITY

The present invention can be used in synchronous transmission between atransmission device and a reception device, is useful in applicationssuch as high definition video transmission over a home network, and isapplicable to stream transmission over a global network such as theInternet.

1-22. (canceled)
 23. A transmission device comprising: a transmissioncounter unit operable to count a constant frequency clock signal; afirst transmission buffer unit operable to temporarily store a packetinput thereto, and output the stored packet; and a transmission unitoperable to (i) transmit the packet stored in the first transmissionbuffer unit and transmission counter information, the transmissioncounter information indicating a count value of the transmission counterunit when the packet was input to the transmission device, and (ii)transmit first transmission storage information via a transmission line,the first transmission storage information specifying a stored amount ofpackets in the first transmission buffer unit at a timing pertaining toprocessing for inputting the packet to the first transmission bufferunit.
 24. The transmission device of claim 23, wherein the firsttransmission storage information is the stored amount of packets in thefirst transmission buffer unit.
 25. The transmission device of claim 23,further comprising: a counter information attachment unit operable toattach the transmission counter information to the packet, wherein thetransmission unit performs the transmission of the packet and thetransmission counter information by transmitting the packet to which thetransmission counter information has been attached by the counterinformation attachment unit.
 26. The transmission device of claim 23,further comprising: a storage information attachment unit operable toattach the first transmission storage information to the packet, whereinthe transmission unit performs the transmission of the packet and thefirst transmission storage information by transmitting the packet towhich the first transmission storage information has been attached bythe storage information attachment unit.
 27. The transmission device ofclaim 23, further comprising: a number attachment unit operable tosequentially attach numbers to packets that are to be transmitted by thetransmission unit, in an order in which the packets were input to thetransmission device.
 28. The transmission device of claim 23, whereinthe first transmission buffer unit outputs the stored packet at aconstant bit rate, the transmission device further comprises: a secondtransmission buffer unit operable to temporarily store the packet outputfrom the first transmission buffer unit, and the transmission unit isfurther operable to transmit second transmission storage information,the second transmission storage information specifying a stored amountof packets in the second transmission buffer unit at a timing pertainingto processing for outputting the packet from the second transmissionbuffer unit.
 29. The transmission device of claim 23, wherein the firsttransmission buffer unit outputs the stored packet at a constant bitrate, and the transmission device further comprises: a secondtransmission buffer unit operable to temporarily store the packet outputfrom the first transmission buffer unit.
 30. A transmission devicecomprising: a transmission counter unit operable to count a constantfrequency clock signal; a first transmission buffer unit operable totemporarily store a packet input thereto, and output the stored packetat a constant bit rate; a second transmission buffer unit operable totemporarily store the packet output from the first transmission buffer;and a transmission unit operable to (i) transmit the packet stored inthe first transmission buffer unit and transmission counter information,the transmission counter information indicating a count value of thetransmission counter unit when the packet was input to the transmissiondevice, and (ii) transmit transmission storage information via atransmission line, the transmission storage information specifying astored amount of packets in the second transmission buffer unit at atiming pertaining to processing for outputting the packet from thesecond transmission buffer unit.
 31. A reception device comprising: areception unit operable to receive, via a transmission line and from atransmission device including a transmission counter unit that counts aconstant frequency clock signal and a first transmission buffer unit,(i) a packet, (ii) transmission counter information indicating a countvalue of the transmission counter unit when the packet was input to thetransmission device, and (iii) first transmission storage informationfor specifying a stored amount of packets in the first transmissionbuffer unit at a timing pertaining to processing for inputting thepacket to the first transmission buffer unit; a reception counter unitoperable to count a clock signal at a counting speed that is variable; afirst reception buffer unit operable to temporarily store the packet; acorrection unit operable to correct the counting speed of the receptioncounter unit based on the first transmission storage information andfirst reception storage information, the first reception storageinformation specifying a stored amount of packets in the first receptionbuffer unit at a timing pertaining to processing for outputting thepacket from the first reception buffer unit; and a first output controlunit operable to control output of the packet from the first receptionbuffer unit based on the transmission counter information received bythe reception unit and reception counter information indicating a countvalue of the reception counter unit.
 32. The reception device of claim31, wherein the first transmission storage information is the storedamount of packets in the first transmission buffer unit, and the firstreception storage information is the stored amount of packets in thefirst reception buffer unit.
 33. The reception device of claim 31,wherein the correction unit corrects the counting speed of the receptioncounter unit so as to obtain a constant value for a sum of the storedamount of packets in the first transmission buffer unit and the storedamount of packets in the first reception buffer unit.
 34. The receptiondevice of claim 31, wherein an input bit rate at which packets are inputto the transmission device is a variable bit rate, and a transmissionbit rate on the transmission line is a constant bit rate.
 35. Thereception device of claim 31, wherein the reception unit is furtheroperable to receive, from the transmission device, sequential numbersthat have been assigned to packets in the transmission device in anorder in which the packets were input to the transmission device, thereception device further comprises: a detection unit operable to detecta count of packets that were not received by the reception unit, basedon the sequential numbers received by the reception unit, and thecorrection unit corrects the counting speed of the reception counterunit so as to obtain a constant value for a sum of the stored amount ofpackets in the first transmission buffer unit, the stored amount ofpackets in the first reception buffer unit, and a missing packet amountthat is based on the detected count of packets that were not received bythe reception unit.
 36. The reception device of claim 31, wherein thereception unit is further operable to receive second transmissionstorage information from the transmission device which further includesa second transmission buffer unit that temporarily stores the packetoutput from the first transmission buffer unit at a constant bit rateand outputs the stored packet, the second transmission storageinformation specifying a stored amount of packets in the secondtransmission buffer unit at a timing pertaining to processing foroutputting the packet from the second transmission buffer unit, and thereception device further comprises: a second reception buffer unitoperable to temporarily store the packet and output the stored packet tothe first reception buffer unit at a constant bit rate; and a secondoutput control unit operable to control the output of the packet fromthe second reception buffer unit, based on the second transmissionstorage information and second reception storage information, the secondreception storage information specifying a stored amount of packets inthe second reception buffer unit at a timing pertaining to processingfor inputting the packet to the second reception buffer unit.
 37. Thereception device of claim 31, wherein the transmission device furtherincludes a second transmission buffer unit that temporarily stores thepacket output from the first transmission buffer unit at a constant bitrate and outputs the stored packet, and the reception device furthercomprises: a second reception buffer unit operable to temporarily storethe packet and output the stored packet to the first reception bufferunit at a constant bit rate.
 38. A reception device comprising: areception unit operable to receive, via a transmission line and from atransmission device including a transmission counter unit that counts aconstant frequency clock signal, a first transmission buffer unit thattemporarily stores a packet and outputs the stored packet at a constantbit rate, and a second transmission buffer unit that temporarily storesthe packet output from the first transmission buffer unit, (i) thepacket, (ii) transmission counter information indicating a count valueof the transmission counter unit when the packet was input to thetransmission device, and (iii) transmission storage information forspecifying a stored amount of packets in the second transmission bufferunit at a timing pertaining to processing for outputting the packet fromthe second transmission buffer unit; a reception counter unit operableto count a clock signal at a counting speed that is variable; a firstreception buffer unit operable to temporarily store the packet; a secondreception buffer unit operable to temporarily store the packet andoutput the stored packet to the first reception buffer unit; acorrection unit operable to correct the counting speed of the receptioncounter unit based on the transmission storage information and receptionstorage information, the reception storage information specifying astored amount of packets in the second reception buffer unit at a timingpertaining to processing for inputting the packet to the secondreception buffer unit; a first output control unit operable to controloutput of the packet from the first reception buffer unit, based on thetransmission counter information received by the reception unit andreception counter information indicating a count value of the receptioncounter unit; and a second output control unit operable to control theoutput of the packet from the second reception buffer unit, based on thetransmission storage information and the reception storage information.39. A transmission method of (i) transmitting a packet stored in a firsttransmission buffer unit and transmission counter information, thetransmission counter information indicating a count value of atransmission counter unit when the packet was input to a transmissiondevice, and (ii) transmitting first transmission storage information viaa transmission line, the first transmission storage informationspecifying a stored amount of packets in the first transmission bufferunit at a timing pertaining to processing for inputting the packet tothe first transmission buffer unit.
 40. A reception method used in areception device which includes a reception counter unit that counts aclock signal at a counting speed that is variable, and a first receptionbuffer unit that temporarily stores a received packet, comprising thesteps of: receiving, via a transmission line and from a transmissiondevice including a transmission counter unit that counts a constantfrequency clock signal and a first transmission buffer unit, (i) thepacket, (ii) transmission counter information indicating a count valueof the transmission counter unit when the packet was input to thetransmission device, and (iii) first transmission storage informationfor specifying a stored amount of packets in the first transmissionbuffer unit at a timing pertaining to processing for inputting thepacket to the first transmission buffer unit; correcting the countingspeed of the reception counter unit based on the first transmissionstorage information and first reception storage information, the firstreception storage information specifying a stored amount of packets inthe first reception buffer unit at a timing pertaining to processing foroutputting the packet from the first reception buffer unit; andperforming first output control to control output of the packet from thefirst reception buffer unit based on the transmission counterinformation received in the receiving step and reception counterinformation indicating a count value of the reception counter unit. 41.A reception method used in a reception device which includes a receptioncounter unit that counts a clock signal at a counting speed that isvariable, a first reception buffer unit that temporarily stores apacket, and a second reception buffer unit that temporarily stores thepacket and outputs the stored packet to the first reception buffer unit,comprising the steps of; receiving, via a transmission line and from atransmission device including a transmission counter unit that counts aconstant frequency clock signal, a first transmission buffer unit thattemporarily stores the packet and outputs the stored packet at aconstant bit rate, and a second transmission buffer unit thattemporarily stores the packet output from the first transmission bufferunit, (i) the packet, (ii) transmission counter information indicating acount value of the transmission counter unit when the packet was inputto the transmission device, and (iii) transmission storage informationfor specifying a stored amount of packets in the second transmissionbuffer unit at a timing pertaining to processing for outputting thepacket from the second transmission buffer unit; correcting the countingspeed of the reception counter unit based on the transmission storageinformation and reception storage information, the reception storageinformation specifying a stored amount of packets in the secondreception buffer unit at a timing pertaining to processing for inputtingthe packet to the second reception buffer unit; performing first outputcontrol to control output of the packet from the first reception bufferunit, based on the transmission counter information received in thereceiving step and reception counter information indicating a countvalue of the reception counter unit; and performing second outputcontrol to control the output of the packet from the second receptionbuffer unit, based on the transmission storage information and thereception storage information.
 42. A reception method used in areception device which includes a reception counter unit that counts aclock signal at a counting speed that is variable, a first receptionbuffer unit that temporarily stores a packet, and a second receptionbuffer unit that temporarily stores the packet and outputs the storedpacket to the first reception buffer unit, comprising the steps of;receiving, via a transmission line and from a transmission deviceincluding a transmission counter unit that counts a constant frequencyclock signal, a first transmission buffer unit that temporarily storesthe packet and outputs the stored packet at a constant bit rate, and asecond transmission buffer unit that temporarily stores the packetoutput from the first transmission buffer unit, (i) the packet, (ii)transmission counter information indicating a count value of thetransmission counter unit when the packet was input to the transmissiondevice, and (iii) transmission storage information for specifying astored amount of packets in the first transmission buffer unit at atiming pertaining to processing for inputting the packet to the firsttransmission buffer unit; correcting the counting speed of the receptioncounter unit based on the transmission storage information and receptionstorage information, the reception storage information specifying astored amount of packets in the first reception buffer unit at a timingpertaining to processing for outputting the packet from the firstreception buffer unit; performing first output control to control outputof the packet from the first reception buffer unit, based on thetransmission counter information received in the receiving step andreception counter information indicating a count value of the receptioncounter unit; and performing second output control to control the outputof the packet from the second reception buffer unit, based on thetransmission storage information and the reception storage information.43. A reception method used in a reception device which includes areception counter unit that counts a clock signal at a counting speedthat is variable, a first reception buffer unit that temporarily storesa packet, and a second reception buffer unit that temporarily stores thepacket and outputs the stored packet to the first reception buffer unit,comprising the steps of; receiving, via a transmission line and from atransmission device including a transmission counter unit that counts aconstant frequency clock signal, a first transmission buffer unit thattemporarily stores the packet and outputs the stored packet at aconstant bit rate, and a second transmission buffer unit thattemporarily stores the packet output from the first transmission bufferunit, (i) the packet, (ii) transmission counter information indicating acount value of the transmission counter unit when the packet was inputto the transmission device, (iii) first transmission storage informationfor specifying a stored amount of packets in the first transmissionbuffer unit at a timing pertaining to processing for inputting thepacket to the second transmission buffer unit, and (iv) secondtransmission storage information for specifying a stored amount ofpackets in the second transmission buffer unit at a timing pertaining toprocessing for outputting the packet from the second transmission bufferunit; correcting the counting speed of the reception counter unit basedon the first transmission storage information and first receptionstorage information, the first reception storage information specifyinga stored amount of packets in the first reception buffer unit at atiming pertaining to processing for outputting the packet from the firstreception buffer unit; performing first output control to control outputof the packet from the first reception buffer unit, based on thetransmission counter information received in the receiving step andreception counter information indicating a count value of the receptioncounter unit; and performing second output control to control the outputof the packet from the second reception buffer unit, based on the secondtransmission storage information and second reception storageinformation, the second reception storage information specifying astored amount of packets in the second reception buffer unit at a timingpertaining to processing for inputting the packet to the secondreception buffer unit.